Phosphate fertilization of soils has always been important. Vast areas of agricultural land can be poor if the soil has a phosphate (P) deficiency. The term available phosphate is used because phosphate is the most immobile of major plant nutrients and if it is not in a soluble form it is difficult if not impossible for plants to get it.

Phosphorus deficiency is generally not as easy to recognize in plants as are deficiencies in many other nutrients. A phosphorus deficient plant is usually stunted, thin-stemmed, and spindly, but its foliage is often dark, almost bluish, green. Thus, unless many larger, healthy plants are present to make a comparison, phosphorus-deficient plants often seem quite normal in appearance. In severe cases, phosphorus deficiency can cause yellowing and senescence of leaves. Some plants develop purple colors in their leaves and stems as a result of phosphorus deficiency, though other related stresses, such as cold tempera­tures, can also cause purple pigmentation. Phosphorus is needed in especially large amounts in meristematic tissues, where cells are rapidly dividing and enlarging.

Phosphorus is very mobile within the plant, so when the supply is short, phosphorus in the older leaves is mobilized and transferred to the newer, rapidly growing leaves. Both the purpling and premature senescence associated with phosphorus deficiency is therefore most prominent on the older leaves. Phosphorus-deficient plants are also characterized by delayed maturity, sparse flowering, and poor seed quality.

Phosphorus in soil

The phosphorus problem in soil fertility is threefold. First, the total phosphorus level of soils is low, usually no more than one-tenth to one-fourth that of nitrogen, and one twentieth that of potassium. The phosphorus content of soils ranges from 200 to 2000 kg phosphorus in the upper 15 cm of 1 ha of soil, with an average of about 1000 kg P. Second, the phosphorus compounds commonly found in soils are mostly unavailable for plant uptake, often because they are highly insoluble. Third, when soluble sources of phosphorus, such as those in fertilizers and manure, are added to soils, they are fixed (changed to unavailable forms) and, in time, form highly insoluble compounds. We will examine these fixation reactions in some detail because they play an important role in determining how much and in what manner phosphorus should be added to soils.

Fixation reactions in soils may allow only a small fraction (10 to 15%) of the phosphorus in fertilizers and manure to be taken up by plants in the year of application. Consequently, farmers who can afford to do so apply two to four times as much phosphorus as they expect to remove in the crop harvest. Repeated over many years, such practices have saturated the phosphorus-fixation capacity and built up the level of available phosphorus in many agricultural soils. Soils having such high levels of soil phosphorus no longer need to be fertilized with more than the amount of phosphorus removed in harvest. In fact, many agricultural soils in industrialized countries with long histories of phosphorus build-up from manure or fertilizer application have accumu­lated so much available phosphorus that little if any additional phosphorus is needed until phosphorus is drawn down to more moderate levels over a period of years. The statistics on fertilizer use in the United States reflect the fact that farmers have recently begun to recognize that fertilizer applications can be reduced where soil phosphorus levels have been built up. The long-term build-up of phosphorus has improved soil fertility, but has also resulted in certain undesirable environmental consequences.

In many developing countries, especially in Africa, such overuse of fertilizer phosphorus is not the rule. In most of sub-Saharan Africa, where per capita food production as been declining in recent years, fertilizer additions of this element for food crops are fraction of the rate of removal of phosphorus in the harvested crops. The soils have been mined of phosphorus for years, with the result that in many areas lack of this element is the first limiting factor in food-crop production. Such phosphorus constraints also indirectly affect the supply of nitrogen, since the growth of most nitrogen-fixing legumes is constrained under low phosphorous conditions. The decline in per capita food production in sub-Saharan Africa will not likely be reversed until the critical phosphorus deficiency problems are solved.

Phosphorus in the environment

Unlike certain nitrogen-containing compounds that are produced during the cycling of nitrogen (e.g., ammonia, nitrates, and nitrosoamines; see Chapter 13), phosphorus added to aquatic systems from soil is not toxic to fish, livestock, or humans. However, too much or too little phosphorus can have severe and widespread negative impacts on environmental quality. The principal environmental problems related to soil phosphorus are land degradation caused by too little available phosphorus and accelerated eutrophication caused by too much. Both problems are related to the role of phosphorus as a plant nutrient.

Many highly weathered soils in the warm, humid, and sub-humid regions of the world have very little capacity to supply phosphorus for plant growth. The low phosphorus availability is partly a result of extensive losses of phosphorus during long periods of rel­atively intense weathering and partly due to the low availability of phosphorus in the aluminum and iron combinations that are the dominant forms of phosphorus in these soils.

Undisturbed natural ecosystems in these regions usually contain enough phosphorus in the biomass and soil organic matter to maintain a substantial standing crop of trees or grasses. Most of the phosphorus taken up by the plants is that released from the decomposing residues of other plants. Very little is lost as long as the system remains undisturbed.

Once the land is cleared for agricultural use (by timber harvest or by forest fires), the losses of phosphorus in eroded soil particles, in runoff water, and in biomass removals (harvests) can be substantial. Within just a few years the system may lose most of the phosphorus that had cycled between the plants and the soils. The remaining inorganic phosphorus in the soil is largely unavailable for plant uptake. In this manner, the phosphorus-supplying capacity of the disturbed soil rapidly becomes so low that regrowth of natural vegetation is sparse and, on land cleared for agricultural use, crops soon fail to produce useful yields.

Leguminous plants that might be expected to replenish soil nitrogen supplies are particularly hard hit by phosphorus deficiency, because low phosphorus supply inhibits effective nodulation and retards the biological nitrogen-fixation process. The spindly plants, deficient in both phosphorus and nitrogen, can provide little vegetative cover to prevent heavy rains from washing away the surface soil. The resulting erosion will fur­ther reduce soil fertility and water-holding capacity. The increasingly impoverished soils can support less and less vegetative cover, and so the degradation accelerates.